The specific surface area of zirconia used as a catalyst support is only about 100 m2/g at 400° C. Zirconia having a specific surface area greater than 100 m2/g is generally amorphous, which does not have a stable structure. When zirconia is used as a catalyst support, its specific surface area is reduced at a temperature higher than 400° C.; thus, it is difficult to obtain stable catalytic performance under high temperature conditions. To use it as a catalyst support, a further improvement in heat resistance (thermal stability) is required.
In addition to heat resistance for maintaining a certain specific surface area, the pore volume distribution, heat resistance of the pores, and the like, have recently been considered important. This is because when precious metals are loaded onto support particles, and when heat treatment is performed, the catalyst support particles are caused to agglutinate, and the number of pores, in particular those having a diameter of 100 nm or more, is greatly reduced while the precious metals, such as platinum, rhodium, and palladium, supported on the surface of the catalyst support particles are embedded inside the particles, and prevented from effectively contributing to the reaction on the surface.
Specifically, precious metals, which are active species as catalysts, are supported with sufficient dispersibility in pores having a diameter of 10 to 100 nm; therefore, it is preferable that the volume of pores having a diameter of 10 to 100 nm be large, and that the volume of pores having a diameter of more than 100 nm be small. It is more preferable that the pores having a diameter of 10 to 100 nm have sufficient heat resistance against high temperatures, so that the large volume of pores having a diameter of 10 to 100 nm are maintained at a temperature as high as 1,000° C. or higher.
Patent Document 1 discloses a catalyst support material having excellent heat resistance. By controlling the particles to have a desired optimum size, this catalyst support material maintains its high specific surface area even when used under a high-temperature atmosphere for a long period of time. The Example of Patent Document 1 discloses, as sample 6, a catalyst support material having a specific surface area of 64.5384 m2/g before heating and 36.7262 m2/g after heating at 1000° C. for 5 hours. However, the heat resistance to maintain a certain specific surface area of this catalyst support material, when converted into the ratio of the specific surface areas of the catalyst support material before and after heating, is only:(A/B)×100=56.9(%).A: specific surface area after heatingB: Specific surface area before heating
Patent Document 2 discloses a zirconia-based porous material having a specific surface area of at least 30 m2/g after heating at 1000° C. for 3 hours. Patent Document 2 further discloses a zirconia-based porous material having a pore diameter peak at 20 to 110 nm in pore diameter distribution by the BJH method, a total pore volume of 0.4 cm3/g or more, and a specific surface area of at least 30 m2/g after heating at 1000° C. for 3 hours.
Patent Document 3 discloses a porous zirconia-based powder having a total pore volume of at least 0.75 mL/g after heat treatment at 1,000° C. for 3 hours. In this porous zirconia-based powder, the total volume of pores having a diameter of 10 to 100 nm after heat treatment at 1,000° C. for 3 hours is at least 30% of the total pore volume. Patent Document 3 also discloses a porous zirconia-based powder having a specific surface area of at least 35 m2/g after heat treatment at 1,000° C. for 3 hours and at least 10 m2/g after heat treatment at 1,100° C. for 3 hours.
Patent Document 4 discloses a zirconium-based composite oxide having a total pore volume of at least 0.35 mL/g after heat treatment at 1,000° C. for 3 hours. In this zirconium-based composite oxide, the pore volume of pores having a diameter of 10 to 100 nm is 0.2 mL/g or more, and the pore volume of pores having a diameter of 100 nm to 10 μm is 0.2 mL/g or less.
As shown in Patent Documents 1 to 4, development is being actively pursued to improve the functions of catalyst support by increasing the pore volume and improving heat resistance to maintain a certain specific surface area. However, all of these cases are evaluated following a short-time heat treatment, and it is not believed that sufficient heat resistance is obtained. Given the above, a catalyst support having more excellent heat resistance has been in demand.
Patent Document 5 discloses an exhaust gas purification catalyst comprising a complex composed of a precious metal and cerium oxide. At least a part of the complex is coated with lanthanum-containing alumina. In this exhaust gas purification catalyst, the pore volume of pores having a diameter of 160 nm or more and less than 1,000 nm is 5% or more and 20% or less of the total pore volume. According to Patent Document 5, the pore volume distribution within the above range prevents performance deterioration caused by high-temperature endurance treatment, and allows exhaust gas to efficiently reach the precious metal, making it possible to more efficiently purify the exhaust gas even after high-temperature endurance. However, the heat resistance is not considered to be sufficient because the pore volume of pores having a diameter of 160 nm or more and less than 1,000 nm is as small as about 20% or less of the total pore volume, and also because the pore volume greatly varies.